A Sound Approach to Pain Relief?

Relief of acute postoperative pain and other forms of acute pain remains a significant challenge. Up to 41 percent of patients experience moderate to severe pain after surgery—a worrying number considering the approximately 234 million major surgical procedures performed globally each year (Weiser et al., 2008). Traditionally, local anesthetics are used in the treatment of acute pain but are often administered as a single, large injection and work only for a relatively brief time. A new study aims to overcome this limitation with a novel delivery platform for local anesthetics that enables on-demand pain relief whose timing, intensity, and duration can be controlled.

The researchers, led by Daniel Kohane, Boston Children’s Hospital, Harvard Medical School, US, used liposomes, which are small sacks of phospholipid molecules surrounding an aqueous solution, to encapsulate the sodium channel blocker and local anesthetic tetrodotoxin (TTX) along with protoporphyrin IX (PPIX); the latter is a “sonosensitizer” that makes the liposome sensitive to effects of sound. Upon exposure to ultrasound, a process known as insonation, PPIX produced reactive oxygen species, which interacted with the liposomal membrane, causing it to degenerate through an oxidative process known as lipid peroxidation. This led to the release of TTX, causing a sustained nerve block in rats.

“This study represents a nice proof of concept of sonosensitization for triggerable drug delivery,” says William Schmidt, NorthStar Consulting, Davis, US, an expert on drug development who was not involved in the study. “There also appears to be relatively rapid onset of analgesia with this technology, which is an important factor in patient-controlled analgesia regimens,” according to Schmidt.

The findings were published online August 9 in Nature Biomedical Engineering, along with an accompanying News and Views by Patrick Couvreur, Paris-Sud University, Chatenay-Malabry, France.

The right trigger

The attractions of triggerable drug delivery systems for pain relief include on-demand controlled release with better analgesia and fewer side effects. In their previous research, Kohane and colleagues encapsulated TTX into liposomes and used near-infrared light to trigger release of the drug (Rwei et al., 2015). However, the depth to which near-infrared light can penetrate into the targeted tissue depends upon the tissue type. This means that how well this approach will work when directed toward nerves would vary based on the location and depth of the target nerve. This limitation could potentially be overcome by increasing how long the tissue is exposed to light, but this runs the risk of causing burns.

The key to sustained release delivery systems is to use a drug with favorable biological characteristics—half-life, absorption, and distribution—as well as a suitable trigger system. “Previously, we have developed liposomal formulations that contain saxitoxin [a sodium channel blocker] and dexamethasone [a glucocorticoid receptor agonist], which will last up to a week after injection. But the problem with these formulations is that once you get nerve block you can’t turn it off,” Kohane told PRF. This can be an issue especially when patients need to move around in order to aid their recovery, since blocking a nerve for too long can affect motor function, too.

An ultrasound strategy

Because ultrasound can penetrate deeper into tissues, be applied in a focused manner, and is commonplace in clinical medicine, it represents a safer, potentially more effective, and easily translatable trigger method.

The authors initially encapsulated PPIX into liposomes along with the fluorescent dye sulforhodamine B (SRho). They confirmed that PPIX could trigger release of liposomal contents by measuring how much SRho was released after insonation, both in vitro and in vivo. Liposomes without PPIX released only a minimal amount of dye after ultrasound.

Further, the peak of SRho released from liposomes correlated with the maximal production of ROS (measured by a fluorescent marker). This strongly indicated that the generation of ROS was the likely mechanism of PPIX-triggered release.

TTX was then encapsulated into liposomes along with PPIX, and a similar release profile was seen after ultrasound, compared to what was observed for SRho release in the initial experiments. Interestingly, release of TTX from liposomes that did not contain PPIX (Lipo-TTX) was faster compared to those that did, suggesting that PPIX is important for the sustained release of TTX. Furthermore, ultrasound applied to the Lipo-TTX formulation five hours after encapsulation did not trigger TTX release. This suggested that PPIX was necessary for the sodium channel blocker to be released upon ultrasound.

Throughout all of these studies the amount of lipid peroxidation (measured by a colorimetric assay) and subsequent SRho or TTX release increased with the frequency, intensity, and duration of ultrasound.

Blocking pain

Next, the authors injected the liposomes at the sciatic nerve of naive male rats and used hindpaw withdrawal latency on the hotplate test as a measure of nerve blockade. The liposome-protoporphyrin IX-tetrodotoxin formulation (Lipo-PPIX-TTX) caused an initial nerve block of approximately eight hours, as shown by increased withdrawal latencies in the animals.

When thermal latencies had returned to baseline levels (latencies of less than four seconds), 10 minutes of ultrasound resulted in a further nerve block lasting about 40 minutes. A second exposure of the same liposomes to ultrasound produced a nerve block lasting only approximately 10 minutes, but no nerve blockade was observed after a third exposure. This was most likely due to “either depletion of the TTX or complete degeneration [due to peroxidation] of the liposome membrane by PPIX,” explained Kohane.

To achieve a more prolonged nerve block, the researchers then co-administered liposomes containing dexmedetomidine (DMED), an α2-adrenergic agonist that prolongs the local anesthetic effects of TTX, alongside the Lipo-PPIX-TTX formulation. Ultrasound produced an initial nerve block lasting approximately 35 hours, and then repeated applications of ultrasound achieved three more nerve blocks lasting about two hours, one hour, and half an hour.

“The addition of DMED liposomes was an important step, as it produces analgesia with a more clinically relevant time frame,” said Kohane.

Histological examination of the sciatic nerve and surrounding tissue revealed only mild inflammation at the injection site and minimal inflammation of surrounding tissues.

An important consideration was to determine if modulation of peripheral nerve conductance by ultrasound alone caused any alteration in hindpaw withdrawal behavior that would indicate nerve block. This would prove that TTX release by ultrasound was the key factor for producing nerve block. To address this question, the authors formulated liposomes without TTX and repeated the same insonation protocol, and also applied ultrasound to non-liposome injected animals, neither of which produced any increases in thermal latencies.

The case for TTX within liposomes

TTX, although not used often in clinical practice, has been under investigation for the treatment of moderate to severe cancer pain and has met with some success (Hagen et al., 2017; Hagen et al., 2011). Furthermore, TTX does have some advantages for the drug delivery approach pursued in the current work.

“TTX is extremely hydrophilic [a strong tendency to dissolve in water; "water loving"], making its encapsulation in liposomes easily achievable in the systems we developed. Other sodium channel blockers such as bupivacaine can also be encapsulated, but these liposome particles cannot be triggered as well due to the amphiphilic nature [both "water loving" and "water hating" properties] of bupivacaine,” according to Kohane.

Although highly effective at blocking nerve conduction, TTX, like many other local sodium channel blockers such as lidocaine, carries a risk of considerable systemic toxicity that has limited its clinical use when injected. Sustained release of TTX using liposomes is one way of overcoming this problem, preventing catastrophic dumping of the drug into the body’s circulation.

Different liposomal formulations of local anesthetics such as bupivacaine exist and are approved for use in patients (Gorfine et al., 2011). But the current data represent a first step in the development of an on-demand analgesic paradigm where patients could control when, how long, and how much drug would be delivered.

“As a proof of concept it is important work, but certainly there are issues to be addressed, in particular the use of more clinically relevant local anesthetics such as bupivacaine or lidocaine, which would strengthen the validity of this approach,” said Schmidt.

As for next steps, Kohane believes that similar nerve block would be observed in injured animals in various rodent models of pain. But the “goal is to move to larger animals as well as resolving some technical issues with the overall aim of moving to human studies.”

Triggerable delivery of anesthetic using non-invasive techniques—especially ultrasound, which is already widely used in medicine—would be a highly desirable approach to pain therapy. For instance: “Imagine if you could ease your pain after a dental filling by shining a small light-emitting diode [LED] over your tooth or by applying ultrasound. The potential applications of this technology are numerous,” said Kohane.

Correction: Because of an editorial error, the second paragraph of the section entitled “Blocking pain” mistated the amount of nerve block with ultrasound after thermal latencies had returned to baseline. It was about .7 hours (approximately 40 minutes), not 7 hours as originally stated. The text has been corrected.